Coolants (quenchants) usable for, for instance, quenching metal components are mainly categorized into an oil-based quenchant and a water-based quenchant (an aqueous solution), the oil-based quenchant being frequently used. Using the oil-based quenchant, which exhibits an appropriate cooling performance, results in a reduced quenching distortion and thus in avoidance of quenching cracks.
However, in quenching a particularly large metal component or a metal component with poor hardenability using the oil-based quenchant, the cooling speed is slow and the cooling performance is insufficient. Accordingly, the water-based quenchant is used. However, in the case of using the water-based quenchant, the cooling performance of which is higher than that of the oil-based quenchant, uneven cooling is likely to occur and thus quenching cracks may be caused.
Quenching with a saline solution has been known for a long time as a technique of improving an uneven-cooling resistance. The saline solution, which does not experience a steam film stage, exhibits a high uneven-cooling resistance as well as a high cooling performance and thus reduces quenching cracks (see Non-Patent Literature 1). However, when the saline solution is used as a quenchant, rust forms in quite a short time of one hour or less after the start of a quenching treatment, resulting in severe corrosion of equipment such as an oil bath. Accordingly, the saline solution is hardly used in an industrial application.
In view of the above, it has been suggested that a water-based quenchant containing an organic acid such as formic acid and acetic acid, an inorganic acid such as hydrochloric acid and sulfuric acid, or a salt thereof is used to improve the cooling speed (see Patent Literatures 1 and 2).